dt-bindings: input: imagis: Document touch keys

IST3032C (and possibly some other models) has touch keys. Document this.

Signed-off-by: Duje Mihanović <duje.mihanovic@skole.hr>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240306-b4-imagis-keys-v3-2-2c429afa8420@skole.hr
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>